The present invention relates to the field of tools run downhole in a borehole. More particularly, the invention relates to an apparatus for transporting a tool downhole through a borehole, and is particularly applicable to deviated and to horizontal boreholes.
Tools are run downhole through boreholes to perform various functions and to identify data relevant to subsurface geologic formations and entrained hydrocarbons. For example, logging tools are run in boreholes to determine the orientation, structure and composition of the borehole and subsurface geologic formations, and to identify the presence of hydrocarbons within the geologic formations. To prevent such tools from becoming stuck within a borehole, such tools are typically run "slick" with a lubricating fluid such as a drilling mud. However, lubricating fluid reduces log quality by interfering with the detection signals generated and received by downhole logging tools.
Roller type "tractors" have been developed to transport tubing and tools downhole through horizontal and deviated wells. Such tractors require multiple moving parts and motors powered with electricity or hydraulic fluid lines. The reliability of downhole tractors is relatively unproven, particularly in uncased boreholes containing fluids. Downhole tractors are relatively expensive and difficult to operate, draw large amounts of power, and have multiple moving parts requiring seals and other maintenance.
Advanced drilling techniques and new completion procedures have increased the complexity of downhole boreholes. Multilateral and horizontal completions shorten the turning radius in deviated wellbores and in the transition between connecting borehole sections. Such boreholes require compact tools which are maneuverable through tight borehole turns and intersections. To navigate narrow boreholes, new tool designs must be smaller than conventional systems. However, the systems must be smaller without reducing the data acquisition and processing capabilities of the tool. Improved downhole tools should preferably be capable of carrying increased instrumentation capabilities and high resolution equipment.
Materials such as shape memory alloys ("SMA") provide actuators for different applications, however SMAs are not conventionally used downhole in boreholes because of operating temperature limitations and useful movement range limitations. SMAs comprise special alloys having the ability to transform from a relatively hard, austenitic phase at high temperature to a relatively flexible, martensitic phase at a lower temperature. SMAs comprise highly thermally sensitive elements which can be heated directly or indirectly to deform the SMA, and can be produced with one-way or two-way memory. An electrical current can resistively heat the SMA to a phase activation threshold temperature by the application of a small electric current through contact leads. Alloy materials providing SMA characteristics include titanium/nickel, copper/zinc/aluminum, and copper/aluminum/nickel compositions.
An SMA in a wire form has two states separated only by temperature. When cool, the SMA is in the martensitic state where the wire is relatively soft and easily deformable. When warmed above the activation temperature, the SMA wire is transformed into the austenitic state wherein the wire is stronger, stiffer and shorter than in the martensitic state. In the martensitic state, an SMA wire is deformed under a relatively low load. When heated above the activation temperature, the SMA wire remembers the original shape and tends to return to such shape. As the SMA wire is heated and contracts, internal stresses opposing the original deformation are created so that the SMA wire can perform work when it returns to the martensitic phase. SMA actuators can use SMA wire in tension as a straight wire or in torsion as a helical wire coil.
The SMA phase transition occurs at a temperature known as the activation temperature. In the low temperature martensitic phase below the activation temperature, the SMA is relatively soft and has a Young's modulus of 3000 Mpa. After the SMA is heated above the activation temperature, the phase transition to a relatively hard austenite phase has a Young's modulus of 6,900 Mpa. If the SMA is not overly deformed or strained, the SMA will return to the original, memorized shape. If the SMA is then cooled, the SMA mechanically deforms to the original martensitic phase. In an SMA formed as a coil spring, heating of the SMA shortens the spring, and cooling the SMA permits the SMA to return to the longer original configuration.
During the manufacture of an SMA, the SMA material is annealed at high temperature to define the structure in the parent, austenitic phase. Upon cooling, the SMA will automatically deflect away from the programmed shape to the configuration assumed by the SMA in the martensitic phase. The SMA can then be alternately heated or cooled with conductive or internal resistance heating techniques to convert the SMA between the austenitic and martensitic phase structures.
As the SMA is heated and cooled, the SMA structurally deflects up to 5%. This deflection can be harnessed with mechanical linkages to perform different work. Although 5% deflection provides a relatively small range of motion, the recovery force can provide forces in excess of 35 to 60 tons per square inch for linear contractions. The rate of mechanical deformation depends of the rate of heating and cooling. In conventional applications, the SMA can be mechanically returned by a restoring force to the configuration of the martensitic shape. This use of a restoring force impacts the geometry and size of mechanisms proposed for a particular use.
SMA materials can be formed into different shapes and configurations by physically constraining the element as the element is heated to the annealing temperature. SMA alloys are available in wire, sheet and tube forms and can be designed to function at different activation temperatures. Large SMAs require relatively high electric current to provide the necessary heating, and correspondingly large electrical conductors to provide high electric current.
Although SMAs are not used downhole in wells because of the limitations described above, SMAs are used in medical devices, seals, eyeglasses, couplings, springs, actuators, and switches. Typically, SMA devices have a single SMA member deformable by heating and have a bias spring for returning the SMA to the original position when cooled. Other actuators termed "differential type actuators" are connected in series so that heating of one SMA deforms the other, and heating of the other SMA works against the first SMA. Representative uses of SMAs are described below.
U.S. Pat. No. 4,556,934 to Lemme et al. (1985) disclosed a shape memory actuator having an end fitting thickness forty percent of the original thickness. The end thickness was reduced so that less current through the end section was required to raise the end temperature above the activation temperature, and the end was cold rolled to strengthen such end against failure.
In U.S. Pat. No. 4,899,543 to Romanelli et al. (1990), a pre-tensioned shape memory actuator provided a clamping device for compressing an object. The actuator comprised a two-way shape memory alloy pre-tensioned to a selected position, and then partially compressed to an intermediate position. The actuator shortened when heated, and then returned to the intermediate clamping position when cooled. The shape memory actuator was formed as a clamping ring or as a coiled spring to accomplish the selected clamping motion.
U.S. Pat. No. 5,127,228 to Swenson (1992) described a shape memory actuator having two concentric tubular shape memory alloy members operated with separate heaters. The torsioned members were engaged at one end so that actuation of one element performed work on the other element, thereby providing a torque density higher than that provided by electromechanical, pneumatic or hydraulic actuators.
U.S. Pat. Nos. 4,979,672 (1990) and 5,071,064 (1991) to AbuJudom et al. disclosed two shape memory alloy elements in the form of a coil spring for operating a damper plate. An electrically conductive rotational connector connected each shape memory element to a control unit and to a stationary member. Each shape memory element was incrementally heated to move a damper plate into intermediate, open and closed positions. U.S. Pat. No. 5,176,544 to AbuJudom et al. (1993) disclosed an actuator having two shape memory elements to control the position of a damper plate. The shape memory elements were shaped as coil springs. One shape memory element moved the damper to an open position, and another shape memory element moved the damper to a closed position.
U.S. Pat. No. 5,445,077 to Dupuy et al. (1995) disclosed a SMA for providing a lock to prevent accidental discharge of a munition. Environmental heating around the munition activated the SMA to operate a munition lock.
U.S. Pat. No. 5,405,337 to Maynard (1995) disclosed a flexible film having SMA actuator elements positioned around a flexible base element. A flexible polyimide film provided the foundation for the SMA actuator elements. Switches were attached with each SMA actuator element, and a microprocessor controller selectively operated the switches and SMA actuator elements to guide the deformation of the base element. U.S. Pat. No. 5,556,370 to Maynard (1996) disclosed an actuator formed with a negative coefficient of expansion material for manipulating a joint. SMA actuators were coiled around a joint to provide three dimensional movement of the joint.
SMAs are limited due to certain operating characteristics. The operable speed of SMAs is limited by the cooling rate of the elements. After the heat source is removed by disconnecting the electrical current or by removing the heat source, the SMA cools through convection or conduction. High temperatures downhole in a well would limit the return of an SMA to the martensitic state. Bias spring actuators do not inherently have two stable positions, and the work output for SMAs per unit volume significantly decreases if the SMAs are used in a bending application. Internally heated SMAs are limited to relatively small cross sections because the current requirements increase with larger cross sectional area. SMA applications are limited by the range of deflection, the deflection of the SMA in a single direction, power requirements, the environmental operating temperatures, and the time required for operation of the SMA.
Conventional downhole locomotion techniques require surface operation of wirelines or tubing, or downhole tractors powered from the surface as described above. Deviated and multilateral wellbore configurations and high operating temperatures challenge conventional techniques for moving a tool downhole in a well. Accordingly, a need exists for improved downhole tools operable within narrow boreholes. Such tools should be compact, inexpensive, and reliable.